bpo-43244: Remove Yield macro from pycore_ast.h (GH-25243)

* pycore_ast.h no longer defines the Yield macro.
* Fix a compiler warning on Windows: "warning C4005: 'Yield': macro
  redefinition".
* Python-ast.c now defines directly functions with their real
  _Py_xxx() name, rather than xxx().
* Remove "#undef Yield" in C files including pycore_ast.h.